The invention is primarily intended for industrial furnaces.

Swedish Patent Specification No. 9800473-2 describes a method and a burner for cleaning waste gases emitted from a gas burner or from some other combustion source, such as oil- fired burners.

It is usual to heat industrial furnaces with the aid of gas burners. The fuel normally used is natural gas, although other gases may be used, such as propane, butane and liquefied petroleum gas or bottled gas.

An example of an efficient gas burner is a burner of the type in which the burner head is placed at one end of an inner pipe and in which the inner pipe is surrounded by an outer protective pipe which is closed at its bottom. The combustion gases from the combustion chamber pass down inside the inner pipe to the bottom of the outer pipe, where they turn to flow back between the outer pipe and the inner pipe and into a waste gas channel which leads to the surroundings. The protective pipe delivers heat to a furnace space by convection, corresponding to 30%, and by radiation, corresponding to 70%.

A similar type of burner also comprises an inner pipe and an outer protective pipe that surrounds the inner pipe, although the bottom of the protective pipe is not closed in this case. The protective pipe is curved, for instance in a U-shaped configuration, and the free end of the protective pipe is connected to a waste gas channel. The inner pipe equipped with the burner head is straight and is thus located within the straight section of the protective pipe.

It is desirable to keep the outer pipe at a temperature of up to 1150-1200°C, so as to thereby enhance the power concentration of the burner. This can be achieved by producing the outer pipe from a high temperature material, such as silicon carbide (SiC) or APM.

APM is an acronym of Advanced Powder Metallurgy, and the material concerned contains about 73% Fe, 22% Cr and 5% Al. The powder material is extruded into a tubular form.

One problem is that when the flame leaves the burner head and comes into contact with the inner pipe, there is formed a region in which the temperature of the inner pipe tends to become too high on relation to the temperature at which the inner pipe is intended to operate.

The present invention solves this problem.

The present invention thus relates to a furnace-heating gas burner of the kind in which the burner head 1 is situated at one end of an inner pipe which is surrounded by an outer protective pipe and wherein a first part of the inner pipe that surrounds the burner head is provided, and wherein a separate inner pipe is provided in the extension of the first pipe which separate inner pipe lies axially in line with the first pipe and commences at the open end thereof, and wherein the burner is characterised in that the outlet of the first pipe has an inner diameter which is smaller than the remainder of the pipe at the end of said first pipe that faces towards the separate pipe.

The invention will now be described in more detail partly with reference to an exemplifying embodiment thereof illustrated in the accompanying drawing, in which - Figure 1 illustrates a burner constructed in accordance with the invention.

Figure 1 illustrates a known type of gas burner for heating furnaces. The gas burner is of the kind in which the burner head 1 is situated at one end of an inner pipe 2, which is surrounded by an outer protective pipe 3. The bottom 4 of the protective pipe 3 is closed.

Thus, the waste gases from the burner head will pass down inside the inner pipe 2 to the bottom 4 of the outer pipe 3 and then turn to flow back between the outer pipe and the inner pipe, as shown by the arrows, and thereafter flow into an exhaust channel 5 that leads to the surroundings.

Figure 1 shows a first part 2 of the inner pipe that surrounds the burner head, and also shows a separate inner pipe 8 situated in the extension of the inner pipe 2. This separate inner pipe 8 is axially in line with the first pipe and commences at the open end of the first pipe 2. The first pipe 2 and the separate inner pipe 8 are thus axially in line with each other. The separate inner pipe 8 commences at the open end of the first pipe 2.

The invention is not restricted to any particular gas burner or to any other type of burner, but may equally as well have been described with reference to the above-mentioned type of burner, which also has an inner pipe that is surrounded by an outer protective pipe, but where the bottom of the protective pipe is not closed but where the protective pipe is curved and the free end of said pipe is connected to a waste gas channel.

Gaseous fuel is fed into an inlet 6 and air of combustion is fed into an inlet 7.

According to the invention, the outlet of the first pipe has a smaller inner diameter than the remainder of the tube at that end which faces towards the separate pipe.

In the Figure 1 embodiment, the first pipe 2 narrows at its lower end. The tapering part 9 causes the speed of the gas to increase towards the mouth 10. In turn, this means that the flame 11 issuing from the mouth will be longer than it would otherwise be when using a typical fully cylindrical first pipe. In turn, the longer flame means that the area of the inner surface of the separate pipe that is heated directly by the flame, i. e. the surface area contacted by the flame, will be much greater than in the case of conventional burners.

It has surprisingly been found that this enlargement of the flame eliminates the aforementioned problem, despite the fact that the flame develops as much heat as in the case of a conventional burner and despite the fact that the inner pipe 8 is heated from both sides.

According to a preferred embodiment of the invention, the smaller inner diameter of the mouth 10 is 10% to 50% of the inner diameter of the remainder of the pipe.

According to another preferred embodiment, the mouth 10 of the first pipe 2 is located essentially at the proximal opening 12 of the separate pipe 8.

Although certain embodiments have been described above, it will be understood that the tapering may have a conical shape or some other shape. It will therefore be understood that the present invention shall not be considered to be restricted to the aforedescribed exemplifying embodiments but that variations can be made within the scope of the accompanying Claims.